Đáp án:

`1)` `S = { - 5/3 }`

`2)` `S = { 3 ; - 7 }`

`3)` `S = { 3/4 ; 1 }`

`4)` `S = { - 5/4 }`

Giải thích các bước giải:

`1)` `- 3x + 2 = 7`

`⇔` `- 3x = 7 - 2`

`⇔` `- 3x = 5`

`⇔` `x = - 5/3`

`Vậy` `S = { - 5/3 }`

`2)` `x ( x - 3 ) + 7x - 21 = 0`

`⇔` `x ( x - 3 ) + 7 ( x - 3 ) = 0`

`⇔` `( x - 3 ) ( x + 7 ) = 0`

`⇔` \(\left[ \begin{array}{l}x-3=0\\x+7=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=3\\x=-7\end{array} \right.\) 

`Vậy` `S = { 3 ; - 7 }`

`3)` `4x^2 - 7x + 3 = 0`

`⇔` `4x^2 - 3x - 4x + 3 = 0`

`⇔` `x ( 4x - 3 ) - ( 4x - 3 ) = 0`

`⇔` `( 4x - 3 ) ( x - 1 ) = 0`

`⇔` \(\left[ \begin{array}{l}4x-3=0\\x-1=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}4x=3\\x=1\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=\dfrac{3}{4}\\x=1\end{array} \right.\)

`Vậy` `S = { 3/4 ; 1 }`

`4)` $\dfrac{x}{x^2-25}$ `-` $\dfrac{3}{x-5}$ `=` $\dfrac{2}{x+5}$          `text{ĐK}` `:` `x` $\neq$ `± 5`

`⇔` $\dfrac{x}{(x-5)(x+5)}$ `-` $\dfrac{3(x+5)}{(x-5)(x+5)}$ `-` $\dfrac{2(x-5)}{(x-5)(x+5)}$ `=0`

`⇔` $\dfrac{x-3x-15-2x+10}{(x-5)(x+5)}$ `=0`

`⇔` $\dfrac{-4x-5}{(x-5)(x+5)}$ `=0`

`⇔` `- 4x - 5 = 0`

`⇔` `- 4x = 5`

`⇔` `x = - 5/4`

`Vậy` `S = { - 5/4 }`